1. FIELD OF THE INVENTION
The present invention relates to seals employed in elastic fluid axial flow turbines and, more particularly, to segmented packing ring seals arranged both where rotatable shafts penetrate stationary turbine casings and, in addition, internal to the casings between stages and turbine sections.
2. DESCRIPTION OF THE PRIOR ART
Generally, such known seals prevent or reduce leakage of the fluid by creating small clearance areas with low flow coefficients between the rotating and stationary parts. Improved efficiency, minimized loss of fluid and prevention of undesirable side effects caused by leakage of fluid are objectives of such seals.
Also, these segmented, labyrinth type seals are vulnerable to rubbing damage caused by turbine misalignment, vibration and thermal distortion. Most of these damage causing factors are more likely to occur during starting, at light loads or following sudden loss of load. As a result, it would be desirable to create a condition of relatively large clearance during these conditions, to minimize possible damage to the seals, and yet still accomplish a small clearance condition at higher loads. The higher load condition corresponds to operation when efficiency is of greatest value and where turbine operation is stable relative to most of the factors which can cause damage to the seals.
It should be recognized that turbine designers already take significant steps to minimize fluid leakage. The seals are made of materials specially selected to minimize damage caused by rubbing. The seal geometry is designed with thin teeth to generate the least amount of heat and force during rubbing situations.
Retractable packing rings which, during start-up conditions, have large radial clearance that automatically decreases to a small clearance condition when a predetermined flow condition has been reached, have also been successfully applied in turbine applications where the pressure forces are significantly greater than the weight forces of the packing segments. In the low pressure stages, however, weight forces are often too large to be successfully overcome by available turbine pressure forces.
In U.S. Pat. No. 5,395,124 issued on Mar. 7, 1995 to Ronald E. Brandon there is disclosed a segmented, retractable, labyrinth-type shaft sealing system for low pressure turbines wherein the packing segments are provided with a combination of gravity or levitation springs that provide a vertical force to neutralize segment weight forces and butt springs that resist pressure forces so as to cause a large clearance at turbine start up and then permit segment motion to allow the clearance to become small after operational conditions are established.
The above patented system has provided satisfactory performance, but with some disadvantages that the subject invention overcomes. These disadvantages include: machining difficulty and expense to properly provide gravity spring holes in the diaphragm or casing and in the packing segments to coincide with the center of gravity of the packing segments; a tendency for the upper half segments to stay in the open or large clearance position until relatively large turbine loads are reached; and some assembly difficulty of springs and segments in the lower half.